Color display devices such as color TVs, color monitors and the like usually represent colors by additive color mixing of RGB primary colors (i.e., red, green and blue). Pixels in a general color display device each include red, green and blue sub pixels in correspondence with the RGB primary colors. By setting the luminance of each of the red, green and blue sub pixels to a desired value, various colors are represented.
The luminance of each sub pixel varies in the range from a minimum gray scale level (e.g., gray scale level 0) to a maximum gray scale level (e.g., gray scale level 255). Herein, for the sake of convenience, the luminance (luminance level) of a sub pixel at the minimum gray scale level is represented as “0”, and the luminance (luminance level) of a sub pixel at the maximum gray scale level is represented as “1”. The luminance (luminance level) of a sub pixel is controlled in the range from “0” to “1”.
When all the sub pixels, namely, the red, green and blue sub pixels have a luminance of “0”, the color displayed by the pixel is black. By contrast, when all the sub pixels have a luminance of “1”, the color displayed by the pixel is white. Many of TV sets available today allow users to adjust the color temperature. In such a case, the color temperature is adjusted by fine-tuning the luminance of each sub pixel. Thus, herein the luminance of a sub pixel after the color temperature is adjusted to a desired level is represented as “1”.
Aside from the above-described display devices using three primary colors, display devices which represent colors by additive color mixing of four or more primary colors have been proposed. Such a display device is referred to as a “multiple primary color display device”. In a multiple primary color display device, the three RGB colors and another color(s) are used. Thus, display can be provided with a wide color reproduction range (see, for example, Patent Documents 1 and 2).
Patent Document 1 discloses a multiple primary color display device in which each of pixels has four or more sub pixels. Patent Document 2 discloses a multiple primary color display device in which each of pixels has red, green, blue, yellow and cyan sub pixels.
With reference to FIG. 26, how the luminance of each sub pixel in the multiple primary color display device disclosed in Patent Document 2 is changed will be described. FIG. 26(a) is a color tone diagram showing a color reproduction range of a pixel in the multiple primary color display device described in Patent Document 2. FIG. 26(b) shows a change of the color displayed by the pixel. FIG. 26(c) shows a change of the luminance of each of the yellow, red, green, cyan and blue sub pixels. In this example, the luminance of each sub pixel is changed such that the color displayed by the pixel is changed from black to white via yellow having an approximately equal hue to the hue of the yellow sub pixel.
In an initial state, the color displayed by the pixel is black, and all the sub pixels have a luminance of Then, the luminance of the yellow sub pixel is increased to “1”. After reaching “1”, the luminance of the yellow sub pixel is kept at “1”.
Next, the luminance of each of the red and green sub pixels is started to be increased. The luminances of the red and green sub pixels are increased to “1” at an equal rate. In this example, the luminances of the red and green sub pixels are increased to “1” at an equal rate, so that the lightness of the pixel is increased without changing the hue. When the luminance of each of the red and green sub pixels reaches “1”, the color displayed by the pixel shows the maximum chroma at this hue, and such a color is referred to also as an “optimal color”. After reaching “1”, the luminance of each of the red and green sub pixels is kept at “1”.
Then, in order to further increase the lightness of the pixel, the luminance of each of the cyan and blue sub pixels is started to be increased. In this example, the luminance of each of the cyan and blue sub pixels is increased while the luminance of each of the red and green sub pixels is kept at “1”, so that the lightness of the pixel is increased without changing the hue. When the luminances of all the sub pixels become “1”, the color displayed by the pixel is white. In this manner, for changing the lightness with the hue of the yellow sub pixel, the multiple primary color display device described in Patent Document 2 starts increasing the luminances of the sub pixels sequentially, namely, from the luminances of the sub pixels having hues closer to the hue of the yellow sub pixel. As a result, the color reproduction range can be expanded.